Recently, as energy saving and carbon dioxide reduction have become a global campaign, and with the awareness of environmental protection on the rise, there is a trend to replace lighting apparatuses using the traditional incandescent or fluorescent lamps with those using light-emitting diodes (LEDs), which consume less power and have a longer service life. In order to ensure that LEDs maintain their design luminosity and thereby enable an LED-based lighting apparatus to provide high-efficiency and high-precision lighting, it is common practice to equip an LED-based lighting apparatus with an LED driving circuit. The LED driving circuit provides a stable output current to the LEDs and thus allows the LEDs to emit light at a fixed luminosity. By contrast, the unstable input voltage of a common driving circuit tends to cause unstable luminosity of LEDs or even generate an excessively high output current that may burn the LEDs.
Conventionally, referring to FIG. 1, the easiest way to design an LED driving circuit is to make a simple constant-current circuit out of a Zener diode DZ1, a PNP transistor Q1, and two resistors R1, R2, among other components. This constant-current circuit generates a fixed output current IO according to Equation (1):
                              I          O                =                                            V              Zener                        -                          V                              EB                ⁡                                  (                  PNP                  )                                                                          R            ⁢                                                  ⁢            1                                              (        1        )            where VZener represents the breakdown voltage of the Zener diode DZ1, and VEB(PNP) represents the emitter-base voltage of the PNP transistor Q1. Thus, an input voltage Input of the constant-current circuit is converted into the fixed output current IO. For example, if the breakdown voltage VZener of the Zener diode DZ1 is 5.1V, R1 is 10Ω, and the emitter-base voltage VEB(PNP) of the PNP transistor Q1 is 0.7 V, then the output current IO of the constant-current circuit is determined by Equation (1) as 440 mA:
      I    O    =                              V          Zener                -                  V                      EB            ⁡                          (              PNP              )                                                  R        ⁢                                  ⁢        1              =                                        5.1            ⁢                                                  ⁢            V                    -                      0.7            ⁢                                                  ⁢            V                                    10          ⁢                                          ⁢          Ω                    =                        0.44          ⁢                                          ⁢          A                =                  440          ⁢                                          ⁢          mA                    Although a constant-current circuit of this kind is advantageously simple in structure and incurs relatively low costs, it is disadvantaged by the fact that both the breakdown voltage VZener of the Zener diode DZ1 and the emitter-base voltage VEB(PNP) of the PNP transistor Q1 vary with temperature, and consequently the output current IO of the constant-current circuit is highly temperature-dependent. Moreover, considerable power loss occurs when there is a large difference between the input voltage and the output voltage of the constant-current circuit, and as a result, the circuit's power utilization efficiency is lowered.
With a view to overcoming the temperature-dependency and low power utilization efficiency of the aforesaid simple constant-current circuit, a constant-current circuit based on the principle of a single-switch isolated flyback converter was developed, as shown in FIG. 2. In the constant-current circuit of FIG. 2, a current sensing element RS is series-connected in the path of the output current IO so that information related to the output current IO can be obtained via the resistance of the current sensing element RS. After the information related to the output current IO is amplified A-fold by a voltage amplifier circuit V-Amp, the amplification result A×RS×IO is input to an error amplifier circuit EA, which compares the amplification result with a reference voltage Vref and generates a control signal accordingly. The control signal is sent to a control circuit CC by way of an optical coupler OC. The control circuit CC switches a power switch Q2 according to the control signal and the following Equation (2), so as to adjust the voltage at the primary winding NP of a transformer T and thereby allow the secondary winding NS to maintain a constant output current IO:
                              A          ×                      I            O                    ×                      R            S                          =                                            V              ref                        →                          I              O                                =                                    V              ref                                      A              ×                              R                S                                                                        (        2        )            For example, if the resistance of the current sensing element RS is 0.1Ω, the voltage amplifier circuit V-Amp has an amplification factor of 100, and the reference voltage Vref is 2.5 V, then the output current IO of the constant-current circuit is determined by Equation (2) as 250 mA:
      I    O    =                    2.5        ⁢                                  ⁢        V                    0.1        ⁢        Ω        ×        100              =                  0.25        ⁢                                  ⁢        A            =              250        ⁢                                  ⁢        mA            However, in spite of overcoming the drawbacks of the aforesaid simple constant-current circuit, the single-switch isolated flyback converter-based constant-current circuit has its own shortcomings, such as a complicated circuit structure and high costs. Furthermore, as the control circuit CC relies on an optical coupling isolation element (i.e., the optical coupler OC) to transmit signals, the overall production costs and circuit layout complexity of the single-switch isolated flyback converter-based constant-current circuit are bound to be much higher than those of the aforesaid simple constant-current circuit.
To eliminate the use of optical coupling isolation elements, another constant-current circuit based on the single-switch isolated flyback converter was developed, as shown in FIG. 3. While the constant-current circuit in FIG. 3 is very similar in structure to that shown in FIG. 2, the former is different from the latter in that the control circuit CC in FIG. 3 must exercise control in accordance with the following three conditions, but there is no such limitation for the constant-current circuit of FIG. 2:    (a) the switching frequency of the power switch Q3 must be kept constant;    (b) the transformer T must be operated in discontinuous conduction mode (DCM); and    (c) the control circuit CC must perform current-mode control.
If the peak value ip-peak of a current ip in the primary winding NP of the transformer T can be fixed while the foregoing three conditions are met, an output current having a fixed power will be generated according to Equation (3):
                              V          ref                =                                                            i                                  P                  -                  peak                                            ×                              R                4                                      →                          i                              P                -                peak                                              =                                    V              ref                                      R              4                                                          (        3        )            where R4 represents resistance, and Vref represents a direct-current (DC) reference voltage. The control circuit CC obtains information related to the current ip in the primary winding NP by way of the resistance R4, such that the information obtained has a voltage waveform of ip×RS. If the peak voltage ip-peak×RS resulting from the current ip in the primary winding NP is equal to the DC reference voltage Vref, then according to Equation (3), the peak value ip-peak of the current ip in the primary winding NP is kept at a fixed value, as shown in FIG. 4. Hence, in order for the constant-current circuit in FIG. 3 to switch from constant-power output to constant-current output and to dispense with the optical coupling isolation element in FIG. 2 (which is configured for obtaining information related to current in the secondary winding), the constant-current circuit in FIG. 3 includes a detection winding NV for obtaining information related to the output voltage VO, wherein the detection winding NV is located on the primary side of the transformer T but in phase with the secondary winding NS. The control circuit CC incorporates the information obtained into its computation so as to determine the peak voltage ip-peak×RS resulting from the current ip in the primary winding NP and thereby keep the output current IO constant to a certain degree.
The constant-current circuit of FIG. 3 is advantageous in that it is not necessary for the control circuit CC to obtain information from the secondary side. This is because the acquisition, computation, and conversion of information are all done on the primary side. Consequently, the constant-current circuit shown in FIG. 3 is much simpler in structure than that shown in FIG. 2 and has been the basis of practically all control ICs on the market that are proclaimed to be designed specifically for LEDs. However, the constant-current circuit in FIG. 3 still has its disadvantages. For instance, since the control circuit CC does not obtain information related to the input voltage, the constant-current circuit of FIG. 3 is applicable only where the input voltage varies within a rather narrow range. Besides, as the control circuit CC causes constant-frequency DCM operation, the power switch Q3 must have certain switching loss. Moreover, when the transformer T is operated in constant-frequency DCM, the energy of the flyback power converter is:PO×TS=½×Lm×iP-peak2  (4)where PO represents output power of the constant-current circuit shown in FIG. 3, TS represents the switching period of the power switch Q3, and Lm represents the magnetizing inductance of the transformer T. According to Equations (3) and (4), the output power PO of the constant-current circuit shown in FIG. 3 can be expressed by Equation (5):
                              P          O                =                                            L              m                        ⁢                          V              ref              2                                            2            ⁢                          T              S                        ⁢                          R              4              2                                                          (        5        )            If both the magnetizing inductance Lm of the transformer T and the resistance R4 are fixed, and given that the constant-current circuit shown in FIG. 3 operates in constant-frequency mode, the switching period TS of the power switch Q3 will also be fixed. Therefore, as long as the DC reference voltage Vref is fixed, the output power PO of the constant-current circuit shown in FIG. 3 will not vary with the input voltage or output voltage. However, referring to FIG. 5 for the waveform of the drain-source voltage of the power switch Q3, when the constant-current circuit of FIG. 3 is operated in constant-frequency DCM, a voltage as high as Vbulk occurs at the instant when the power switch Q3 is turned on. As the power switch Q3 consumes all the voltage Vbulk, the power switch Q3 tends to generate heat upon conduction. Hence, when the constant-current circuit, of FIG. 3 is operated in constant-frequency DCM, a huge power loss takes place in the power switch Q3 at the instant when it is turned on, thus causing unnecessary waste of electric power.
Commercially available constant-current circuits are typically application circuits derived from the basic structure of the constant-current circuit shown in FIG. 3. For example, the constant-current circuit illustrated in FIG. 6 uses a current control IC which is made by Fairchild Semiconductor and sold under the Model No. FSEZ1016A, the constant-current circuit in FIG. 7 uses a current control IC made by Fairchild Semiconductor and sold under the Model No. FAN103, and the constant-current circuit in FIG. 8 uses a current control IC made by Power Integrations and sold under the Model No. LNK605DG. When these constant-current circuits are operated in constant-frequency DCM, their power switches are all subject to heat generation and undue power loss upon being turned on.
Therefore, it is imperative to overcome the foregoing drawbacks of the prior art and develop a constant-current circuit which has a simple circuit layout with relatively few electronic elements, can do without any secondary constant-current detection circuits or feedback circuits, and yet allows all the necessary information to be obtained from the primary side for computation and execution, so as to generate an output current that remains constant over a wide range of input voltage; and in which the switching loss of power semiconductor elements (including the primary-side power switch and the secondary-side current rectifying element) is effectively reduced to increase power conversion efficiency.